TETRANUCLEAR HETEROMETALLIC COMPLEXES OF THE GENERAL TYPES [M((OH)2CRA4)3]5+ AND [M((OH)2COA4)3]5+

Series of tetranuclear heteronuclear complex ions of the general types [M{(OH)2CrA4}3]5+ and [M{(OH)2CoA4}3]5+, where M is a divalent metal ion and A4 is a single tetradentate amine ligand, two bidentate amines, or four monodentate amines, have been synthesized and characterized. The structure of the cobalt(II)/chromium(III) complex [Co{(OH)2Cr(bispictn)}3](ClO4)5.6H2O (where bispictn is N,N'-bis(2-pyridylmethyl)-1,3-propanediamine, C15H20N4) has been determined by single-crystal X-ray diffraction techniques. The complex crystallizes in the acentric space group P31c of the trigonal system with two tetranuclear species in a cell of dimensions a = 16.144 (5) angstrom and c = 15.091 (2) angstrom. The structure has been refined to a value of the weighted R factor of 0.046 based on 1604 independent observed reflections. The Co atom lies on the 3-fold axis and is surrounded by six bridging hydroxo groups. Hence, the complex can be viewed as consisting of three cis-[Cr(bispictn)(OH)2]+ units, which act as bidentate ligands to the central cobalt(II) core. The Co-O bond lengths are 2.050 (5) and 2.166 (5) angstrom, while the Cr-O distances are 1.902 (5) and 1.921 (5) angstrom. The Co-Cr distance in the tetramer is 3.064 (2) angstrom, comparable to the values in bis(mu-hydroxo)dichromium(III) dimers. The magnetic properties of the complexes have been investigated and are compared with the theoretical values obtained by fitting to the appropriate Hamiltonian. General energy diagrams illustrating the relation among energy, spin, and degeneracy for tetranuclear systems having S1 between 1/2 and 5/2, S2 = S3 = S4 = 3/2. J12 = J13 = J14 = J, and J23 = J34 = J42 = 0 are given; these diagrams illustrate that in all cases, independent of S1 and the sign of J, the ground state has a higher spin than the nearest excited states. This has important consequences for the variation of the effective magnetic moments with the temperature.